1. Field of the Invention
The invention relates to a method for polymerizing alpha-olefins, a catalyst for such a polymerization method and a method for producing such a catalyst. More particularly, the invention relates to a vapor-phase method for preparing a polymer of one or more alpha-olefins which produces linear low density polyethylene (LLDPE) or high density polyethylene (HDPE) at relatively low partial pressures of the reactants.
2. Description of the Prior Art
Linear low density polyethylene polymers possess properties which distinguish them from other polyethylene polymers, such as homopolymers of polyethylene. Certain of these properties are described in Anderson et al., U.S. Pat. No. 4,076,698.
Karol et al., U.S. Pat. No. 4,302,566, describes a process for producing certain linear low density polyethylene polymers in a gas phase, fluid bed reactor.
Graff, U.S. Pat. No. 4,173,547, Stevens et al., U.S. Pat. No. 3,787,384, Strobel et al., U.S. Pat. No. 4,148,754, and Ziegler, deceased, et al., U.S. Pat. No. 4,063,009, each describe various polymerization processes suitable for producing forms of polyethylene other than linear low density polyethylene, per se.
Graff, U.S. Pat. No. 4,173,547, describes a supported catalyst obtained by treating a support with both an organoaluminum compound and an organomagnesium compound followed by contacting this treated support with a tetravalent titanium compound.
Stevens et al., U.S. Pat. No. 3,787,384, and Strobel et al., U.S. Pat. No. 4,148,754, describe catalysts prepared by first reacting a support (e.g., silica containing reactive hydroxyl groups) with an organomagnesium compound (e.g., a Grignard reagent) and then combining this reacted support with a tetravalent titanium compound. According to the teachings of both of these patents, no unreacted organomagnesium compound appears to be present when the reacted support is contacted with the tetravalent titanium compound.
Ziegler, deceased, et al., U.S. Pat. No. 4,063,009, describes a catalyst which is the reaction product of an organomagnesium compound (e.g., an alkylmagnesium halide) with a tetravalent titanium compound. The reaction of the organomagnesium compound with the tetravalent titanium compound takes place in the absence of a support material.
A vanadium-containing catalyst, used in conjunction with triisobutylaluminum as a co-catalyst, is disclosed by W. L. Carrick et al in Journal of American Chemical Society, Volume 82, page 1502 (1960) and Volume 83, page 2654 (1961).
Nowlin et al., U.S. patent application Ser. No. 444,152, filed Nov. 24, 1982, now U.S. Pat. No. 4,481,301, teach an alpha-olefin polymerization catalyst prepared in a multi-step process. First, a solid, porous carrier containing reactive hydroxyl groups (OH) is reacted with a greater than a stoichiometric amount of an organomagnesium composition. The product of that step is then reacted with a tetravalent titanium compound, also used in the amount greater than the stoichiometric amount thereof with respect to the hydroxyl groups on the carrier, to produce a catalyst precursor. The catalyst precursor is then combined with an activator, also known as a co-catalyst, to produce an active catalyst composition.
Gas phase alpha-olefin polymerization processes were carried out in the prior art at relatively high partial pressures of ethylene and other monomers. For example, the gas phase process of Karol et al is normally conducted at a partial pressure of ethylene of about 120 psi and; when 1-butene is used as a comonomer, at a partial pressure ratio of 1-butene/ethylene of about 0.1-0.5. At such a relatively high partial pressure of ethylene, a substantial portion of the reactants undergoes side hydrogenation reactions which produce alkanes (e.g., ethylene to ethane). In addition, the relatively high reactants' partial pressure contributes to the incorporation of relatively large amounts of monomers, in an unreacted form, into the polymer products. Such monomers do not become incorporated into the polymer chain, do not contribute to the properties thereof and therefore constitute "lost" reactants. Both of these factors increase the cost of the polymerization reaction and of the resulting product.
It is a primary object of the present invention to provide an active catalyst composition which is capable of polymerizing alpha-olefins in a continuous, gas phase fluid bed reactor at relatively low partial pressures of the reactants.
It is an additional object of the invention to provide a continuous, gas phase catalytic process for polymerizing alpha-olefins which yields linear low density or high density polyethylene or a copolymer of ethylene and at least one C.sub.3 -C.sub.10 olefin at relatively low partial pressures of the reactants.